Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 257
Q. 8. Define normal hydrogen electrode.
Ans. It is a reference or standard reference electrode. Its electrode potential is taken as
zero at all temperatures. A normal hydrogen electrode generally consists of a Pt-foil coated
with platinum, dipped in solution having 1 M H + ion concentration and hydrogen gas at
1 atmospheric pressure constantly bubbled over it. It can be represented as:
Pt, H 2
(1 atm) || H + (1 M)
Reaction: H + (aq) + e – ⎯⎯→ 1 2 H 2 ( g)
or 1 2 H 2 ( g ) ⎯⎯→ H+ (aq) + e –
Q. 9. How is electrode potential measured?
Ans. Apparatus for measuring the standard electrode potential of a redox system is
given in Fig. 10.17.
H2
(1 atm)
Saline bridge
Inert
electrode (Pt)
H + (a=1)
Pt
Redox system
A
B
Fig. 10.17 A cell constructed with standard hydrogen electrode.
A = Solution of H + ions of unit activity
B = Redox solution of unit activity.
Saline bridge has the function of bringing into physical contact the two solutions without
mixing them; it consists of a strong electrolyte (e.g., KCl).
Q. 10. How is the E.M.F. of a cell measured?
Ans. E.M.F. of a cell is measured by means of a device, named potentiometer.
∆∈ = ∈° Redox
– ∈° +
∴ ∈° +
H2 / H
= 0
H2 / H
∴ ∆∈ = ∈° Redox
B
A
D
C
Fig. 10.18 Diagram for Potentiometer.
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